NullPointerException

Question

\ Problem is about the NullPointerException that appears in the WifiScanner class while detecting bestsignal.

java.lang.RuntimeException: Unable to start activity
ComponentInfo{project.e.wifiactivity/project.e.wifiactivity.WifiConnector}:
java.lang.NullPointerException: Attempt to invoke virtual method
'java.lang.String android.net.wifi.ScanResult.toString()'

\

 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method
'java.lang.String android.net.wifi.ScanResult.toString()' on a null object reference
        at
project.e.wifiactivity.WifiScanner.bestReturn(WifiScanner.java:46)
        at
project.e.wifiactivity.WifiConnector.onCreate(WifiConnector.java:23)

\Logcat

03-30 04:26:07.351  26037-26037/? I/art﹕ Late-enabling -Xcheck:jni  
03-30 04:26:07.522  26037-26059/project.e.wifiactivity D/OpenGLRenderer﹕ Use EGL_SWAP_BEHAVIOR_PRESERVED: true  
03-30 04:26:07.525  26037-26037/project.e.wifiactivity D/﹕ HostConnection::get() New Host Connection established 0xf3e5c7b0, tid 26037  
03-30 04:26:07.536  26037-26037/project.e.wifiactivity D/Atlas﹕ Validating map...  
03-30 04:26:07.573  26037-26059/project.e.wifiactivity D/libEGL﹕ loaded /system/lib/egl/libEGL_emulation.so  
03-30 04:26:07.574  26037-26059/project.e.wifiactivity D/libEGL﹕ loaded /system/lib/egl/libGLESv1_CM_emulation.so  
03-30 04:26:07.596  26037-26059/project.e.wifiactivity D/libEGL﹕ loaded /system/lib/egl/libGLESv2_emulation.so  
03-30 04:26:07.607  26037-26059/project.e.wifiactivity D/﹕ HostConnection::get() New Host Connection established 0xeec393f0, tid 26059  
03-30 04:26:07.628  26037-26059/project.e.wifiactivity I/OpenGLRenderer﹕ Initialized EGL, version 1.4  
03-30 04:26:07.672  26037-26059/project.e.wifiactivity D/OpenGLRenderer﹕ Enabling debug mode 0  
03-30 04:26:07.692  26037-26059/project.e.wifiactivity W/EGL_emulation﹕ eglSurfaceAttrib not implemented  
03-30 04:26:07.692  26037-26059/project.e.wifiactivity W/OpenGLRenderer﹕ Failed to set EGL_SWAP_BEHAVIOR on surface 0xeec38740, error=EGL_SUCCESS  
03-30 04:26:09.662  26037-26059/project.e.wifiactivity W/EGL_emulation﹕ eglSurfaceAttrib not implemented  
03-30 04:26:09.662  26037-26059/project.e.wifiactivity W/OpenGLRenderer﹕ Failed to set EGL_SWAP_BEHAVIOR on surface 0xf3d0cc60, error=EGL_SUCCESS    
03-30 04:26:27.858  26037-26037/project.e.wifiactivity D/WifiDemo﹕ onCreate()  
03-30 04:26:27.936  26037-26059/project.e.wifiactivity W/EGL_emulation﹕ eglSurfaceAttrib not implemented  
03-30 04:26:27.936  26037-26059/project.e.wifiactivity W/OpenGLRenderer﹕ Failed to set EGL_SWAP_BEHAVIOR on surface 0xf3e840a0, error=EGL_SUCCESS    
03-30 04:26:28.130  26037-26037/project.e.wifiactivity D/AndroidRuntime﹕ Shutting down VM  
03-30 04:26:28.130  26037-26037/project.e.wifiactivity E/AndroidRuntime﹕ FATAL EXCEPTION: main  
Process: project.e.wifiactivity, PID: 26037  
    java.lang.RuntimeException: Unable to start activity ComponentInfo{project.e.wifiactivity/project.e.wifiactivity.WifiConnector}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.net.wifi.ScanResult.toString()' on a null object reference  
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)  
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)  
            at android.app.ActivityThread.access$800(ActivityThread.java:151)  
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)  
            at android.os.Handler.dispatchMessage(Handler.java:102)  
            at android.os.Looper.loop(Looper.java:135)  
            at android.app.ActivityThread.main(ActivityThread.java:5254)  
            at java.lang.reflect.Method.invoke(Native Method)  
            at java.lang.reflect.Method.invoke(Method.java:372)  
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)  
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)  
     **strong text**Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.net.wifi.ScanResult.toString()' on a null object reference**  
            at project.e.wifiactivity.WifiScanner.bestReturn(WifiScanner.java:46)  
            at project.e.wifiactivity.WifiConnector.onCreate(WifiConnector.java:23)  
            at android.app.Activity.performCreate(Activity.java:5990)  
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)  
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)  
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)  
            at android.app.ActivityThread.access$800(ActivityThread.java:151)  
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)  
            at android.os.Handler.dispatchMessage(Handler.java:102)  
            at android.os.Looper.loop(Looper.java:135)  
            at android.app.ActivityThread.main(ActivityThread.java:5254)  
            at java.lang.reflect.Method.invoke(Native Method)  
            at java.lang.reflect.Method.invoke(Method.java:372)  
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)  
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

\MainActivity class /package project.e.wifiactivity;

    import android.content.BroadcastReceiver;
    import android.content.Context;
    import android.content.Intent;
    import android.content.IntentFilter;
    import android.net.wifi.WifiConfiguration;
    import android.net.wifi.WifiInfo;
    import android.net.wifi.WifiManager;
    import android.os.Bundle;
    import android.support.v7.app.AppCompatActivity;
    import android.util.Log;
    import android.view.View;
    import android.widget.Button;
    import android.widget.TextView;
    import android.widget.Toast;

    import java.util.List;

    public class MainActivity extends AppCompatActivity implements View.lickListener {

    private static final String TAG="WifiDemo";
    WifiManager wifi;
    BroadcastReceiver receiver;
    public Login login = new Login();




    TextView text;
    Button btn;
    Button buttonp;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        text=(TextView)findViewById(R.id.text);
        btn=(Button)findViewById(R.id.btn);
        Button enb = (Button) findViewById(R.id.enb);    v

        final  WifiManager wifiManager = (WifiManager) getSystemService(Context.WIFI_SERVICE);

           enb.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (!wifiManager.isWifiEnabled()) {
                    wifiManager.setWifiEnabled(true);
                    Toast.makeText(MainActivity.this, "Wifi Enabled", Toast.LENGTH_LONG).show();

                }
            }

        });

        buttonp = (Button) findViewById(R.id.buttonp);
        buttonp.setOnClickListener(this);
        Intent l = new Intent(this, WifiConnector.class);
        startActivity(l);







        btn.setOnClickListener(this);
        //get wifi status
        wifi=(WifiManager)getSystemService(Context.WIFI_SERVICE);
        WifiInfo info=wifi.getConnectionInfo();
        String name = info.getSSID();
        String macid = info.getMacAddress();
        text.append("\n\nWifi Name :"+ name +"\nMac Id :"+ macid);
        //list available network

        List<WifiConfiguration> configurations= wifi.getConfiguredNetworks();
        for(WifiConfiguration configuration : configurations){
            text.append("\n\n" +configuration.SSID+"\n"+configuration.BSSID);
        }

        //register broadcast receiver

        if(receiver==null) {
            receiver = new WifiScanner();
            registerReceiver(receiver, new IntentFilter(WifiManager.SCAN_RESULTS_AVAILABLE_ACTION));
            Log.d(TAG, "onCreate()");
        }




    }









    @Override
    protected void onStop() {
        try {
            this.unregisterReceiver(receiver);
            super.onStop();
        } catch (IllegalArgumentException f) {
            f.printStackTrace();
        }
    }

    @Override
    protected void onPause() {


        try {
            super.onPause();
            unregisterReceiver(receiver);
        } catch (IllegalArgumentException e) {
            e.printStackTrace();
        }
    }
    @Override
    protected void onResume() {


        super.onResume();

        IntentFilter intentFilter = new IntentFilter();
        intentFilter.addAction("android.net.conn.CONNECTIVITY_CHANGE");
        registerReceiver(receiver, intentFilter);


        int f = login.resultp();

        if (f == 1) {
            Intent l = new Intent(this, MainActivity.class);
            startActivity(l);

        }
    }




















    @Override
    public void onClick(View view) {
    Toast.makeText(getApplicationContext(), "All Network Searched!!", 0).show();
        if (R.id.btn == view.getId()){
             Log.d(TAG, "onCreate() wifi.startScan()");
             wifi.startScan();

        }
    }
}

\Wifi Scanner Class

package project.e.wifiactivity;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.net.wifi.ScanResult;
import android.net.wifi.WifiManager;
import android.util.Log;
import android.widget.Toast;

import java.util.List;


public class WifiScanner extends BroadcastReceiver {


public ScanResult bestsignal ;
private static final String TAG="WifiScanReceiver";
MainActivity main;




@Override
public void onReceive(Context arg0, Intent arg1) {

     List<ScanResult> results = main.wifi.getScanResults();


    for(ScanResult result : results) {
        if(bestsignal == null || WifiManager.compareSignalLevel(bestsignal.level, result.level)<0)
        bestsignal = result;
        bestReturn();


    }
     String message=String.format("%s networks found.%s is the strongest.", results.size(), bestsignal.SSID);
    Toast.makeText(main, message, 0).show();
    Log.d(TAG, "onReceive() message :"+message);


}

public String bestReturn() {

    return bestsignal.toString();

}

} \Wifi Best Signal Connector Class

package project.e.wifiactivity;


import android.app.Activity;
import android.content.Context;
import android.net.wifi.WifiConfiguration;
import android.net.wifi.WifiManager;
import android.os.Bundle;

import java.util.List;



public class WifiConnector extends Activity {



    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        WifiScanner wifiscannner = new WifiScanner();
        String bestnetSSID = wifiscannner.bestReturn();
        String networkPass = "pass";

        WifiConfiguration conf = new WifiConfiguration();
        conf.SSID = "\"" + bestnetSSID + "\"";

        conf.preSharedKey = "\"" + networkPass + "\"";

        WifiManager wifiManager = (WifiManager) getSystemService(Context.WIFI_SERVICE);
        wifiManager.addNetwork(conf);

        List<WifiConfiguration> list = wifiManager.getConfiguredNetworks();
        for (WifiConfiguration i : list) {
            if (i.SSID != null && i.SSID.equals("\"" + bestnetSSID + "\"")) {
                wifiManager.disconnect();
                wifiManager.enableNetwork(i.networkId, true);
                wifiManager.reconnect();

                break;
            }
        }

    }


}

\Manifest

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="project.e.wifiactivity" >
<uses-feature android:name="android.hardware.wifi" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.CHANGE_WIFI_STATE" />
<uses-permission android:name="android.permission.UPDATE_DEVICE_STATS" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
<uses-permission android:name="android.permission.CHANGE_NETWORK_STATE"/>


<application
    android:debuggable="true"
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".Display"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity android:name=".Login"></activity>
    <activity android:name=".MainActivity"></activity>
    <activity android:name=".WifiConnector"></activity>


</application>

</manifest>

the error is because of null object...your ScanResult is null — H Raval, Mar 30 '16 at 10:05
I understand that but what could be the proper way of initializing ScanResult, I want to know that. — user3537196, Mar 30 '16 at 10:07

Rohit Arya · Accepted Answer · 2016-03-30T10:11:59.553

1

Quick fix:

public String bestReturn() {
  if(bestsignal!=null)
    return bestsignal.toString();
  else{
    return null;
  }
}

Problem:

You are calling this

String bestnetSSID = wifiscannner.bestReturn();

which creates an instance of BroadcastReceiver and thus it's onReceive callback hasn't been called yet. Thus bestsignal remains null.

edited Mar 30 '16 at 10:11

answered Mar 30 '16 at 10:06

Rohit Arya

6,751
1
26
40

score 0 · Answer 2 · answered Mar 30 '16 at 10:08

0

Intent l = new Intent(this, MainActivity.class);
        startActivity(l);   
" this = MainActivity.this "

MainActivity not start to MainActivity .. :D

answered Mar 30 '16 at 10:08

Jacky Chan

53
1
7

was testing around, must have happened in the confusion... :-) anyways thanks,, :-) – user3537196 Mar 30 '16 at 10:11

NullPointerException

2 Answers2